runtime: mips32 soft float point support

[juliandroid]

I have built Golang by using the master branch and I have realized that there is no soft point support for the new mips32 target. That means the core like MIPS32 74Kc is not supported due to lack of FPU.

I have wrote an email to Vladimir Stefanovic, asking him to confirm the soft-float point support issue and his response was that in the near future there are no plans soft-point support to be implemented, but that might change later.

It would be great to have a ticket here to keep track the development of the issue and hopefully one day this issue to be resolved.

[bradfitz]

/cc @vstefanovic @cherrymui

[cherrymui]

see also #14635.

[cherrymui]

There are softfloat code for ARM that we can reuse. What still need to be done is MIPS instruction decoding.

[vstefanovic]

@juliandroid
Yes, and after you quite a few other people asked about soft float support, so looks like the future has almost arrived.
If the code wasn't in feature freeze state, the patch would be submitted relatively soon.

[juliandroid]

@vstefanovic
That was fast, the issue comes with twist :) Thanks for working on it!
Can you please leave the commit id (gerrit) here, once you complete the soft-float support? I'm willing to try it even before the merge in the master.

[vstefanovic]

@juliandroid
Sure, I'll add a link.

[minux]

After discussion with @cherrymui on this issue, we think it's probably a good idea to always enable soft float for mips and mips64 (for both endianness) so that we don't need to add a GOMIPS knob. We can make _sfloat return immediately without interpreting the following FPU instructions if the machine does have an FPU. The rationale is that most MIPS machines do not have an FPU, this is unlike the ARM situation where if we do drop ARMv6 support, almost all supported machine will have an FPU. Of course, we should base our final decision on both binary size and execution time overhead for this. If we decide to add a $GOMIPS knob to mirror the existing $GOARM, we should also consider add support for the legacy 32-bit MIPS instruction sets (i.e. MIPS I/II) so that our 32-bit port also runs on Loongson 2.

[petar-jovanovic]

The rationale is that most MIPS machines do not have an
FPU

I have to disagree. Majority of modern MIPS boards do have FP
unit. There is a long list of legacy boards that indeed lack FPU
support, but relatively modern MIPS boards all have it.
$GOMIPS is important also to make a distinction between MIPS32r1/r2/r6 and
MIPS64r2/r6 (and MIPS3 as a legacy) that we plan to support through Go.

[aep]

@vstefanovic any progress? need any help with this?

[vstefanovic]

@aep
Thanks, this will be submitted after go 1.8 is released, since it was too late for 1.8.

[elimisteve]

@vstefanovic I just heard of this via https://groups.google.com/forum/#!topic/golang-nuts/DT_P9UPKHgI . Adding soft floating point support for MIPS(32) would be fantastic! As soon as a patch or release is ready, I'm excited to use it. Think that might be soon?

[vstefanovic]

@elimisteve great to hear that. I'd like to prepare docker diffs first, then get back to the soft-float with a rebase and some double-checking; presumably it'll be uploaded in a few weeks.

[juliandroid]

I'm watching the calendar on the wall patiently too 👍

[krasin]

@vstefanovic any updates? I would be willing to be a guinea pig for any work-in-progress patch you might have. The device I would like to run Go binaries on is based on MT7628AN, mips32 little endian, if that matters.

[vstefanovic]

Hi @krasin, give me a few more days, then I'll upload the code to gerrit.

[krasin]

@vstefanovic sure thing. As a workaround, I have compiled my kernel with FPU emulation flag, but that does not feel like the way to go, so a proper soft float support is highly appreciated.

[gopherbot]

CL https://golang.org/cl/37954 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37955 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37958 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37956 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37957 mentions this issue.

[pqyptixa]

@vstefanovic
I cloned Go master and merged your patches, but still wasn't able to run the binary:

$ echo -e 'package main\nfunc main(){\n\tprintln("hello world!")\n}' > hello.go
$ env GOOS=linux GOARCH=mips GOMIPS=soft-float,mips32r2 ~/go-mips32-soft-float/bin/go build hello.go
scp hello my@mips:/tmp/

At mips system (openwrt):

# /tmp/hello 
Illegal instruction
# gdb /tmp/hello
[...]
(gdb) x/i $pc 
=> 0x4732c <runtime.check+48>:  mtc1    zero,$f0

Compiling with/without GOMIPS=soft-float,mips32r2 doesn't seem to make any difference:

$ env GOOS=linux GOARCH=mips GOMIPS=soft-float,mips32r2 ~/go-mips32-soft-float/bin/go build hello.go
dacf38f5eb593a1e145f47de0da08c4e  hello
$ rm hello
$ env GOOS=linux GOARCH=mips ~/go-mips32-soft-float/bin/go build hello.go
$ md5sum hello
dacf38f5eb593a1e145f47de0da08c4e  hello
$ file hello
hello: ELF 32-bit MSB executable, MIPS, MIPS32 version 1 (SYSV), statically linked, not stripped

However, the go compiler is parsing the GOMIPS env var:

$ env GOOS=linux GOARCH=mips GOMIPS=blah ~/go-mips32-soft-float/bin/go build hello.go
# command-line-arguments
[...] Invalid GOMIPS value. Valid values are: soft-float/hard-float, mips32/mips32r2.
If more than one, separate them with comma. E.g. GOMIPS=soft-float,mips32.

Am I doing something wrong, or missing something? Are these patches not yet complete?

BTW, thanks a lot for your work!

[vstefanovic]

@pqyptixa
The patches are complete. Maybe you're missing '-a', ie. 'go build -a', if you have previously built something as hard-float.
The build system doesn't do clean build when you change GOMIPS (or GOARM, GO386) value, we might want to improve that, but at the moment 'go build -a' is the safe way.

[pqyptixa]

@vstefanovic
That was it, I needed to use -a ! :D
Thank you!

[ALTree]

@petar-jovanovic I pinged them yesterday asking for an update about the CLs and Cherry said they were all reviewed except this one: https://go-review.googlesource.com/c/go/+/37954. So as soon as that one is also +2ed reviewers will decide if they can be all merged to be included in 1.10.

[bradfitz]

I'm bumping this back to Go 1.10 because it might've been moved to Go1.11 along with a bunch of other bugs en masse.

I'd prefer this go into Go 1.10 if @rsc doesn't find it too invasive.

[juliandroid]

I would greatly appreciate if this ends up in Go 1.10 as @aclements pledged to try his best last time :)

@bradfitz Thanks for pushing this into 1.10 👍

[aclements]

This is fixed as of 6be1c09. Thanks! I opened #22966 for creating a builder to test this configuration.

[dtodor]

We have a problem with the Go compiler on a MIPS target without the CONFIG_MIPS_FPU_EMULATOR being enabled. I thought in such case the soft-float support in go1.10RC1 would be sufficient but unfortunately be cannot build any runnable Go programs. The FPU configuration of the target is:
CONFIG_MIPS_FPU_EMULATOR=n
CONFIG_CPU_R4K_FPU=y

Any ideas?

[davecheney]

@dtodor. Please open a new issue including how you built Go or the target binary.

[wengerxin]

Is soft point support for mips64 ready?

[ALTree]

@wengerxin No, apparently. Softfloat for mips64 is issue #14635, which is still open.

[cherrymui]

@wengerxin Soft float on MIPS64 are not officially supported. However, the functionality are all there. You are welcomed to try it, by setting GO_GCFLAGS="-d softfloat" on bootstrap and setting -gcflags=all="-d softfloat" on go build. (Note: this is not tested though, and assembly code, if any, are not covered.)

[wengerxin]

I consulted the mips32 modification and modified the flowing files to shield the float instructions:
src/runtime/cgo/asm_mips64x.s
/src/runtime/cgo/gcc_mips64x.S

0001-add-mips64-soft-float.zip
Modifications are in the attached file, but I don't know whether these modifications are right?

[milanknezevic]

@wengerxin These modifications are needed only if you need CGO (you can disable it with CGO_ENABLED=0). But if you need it, you are on the right track. Are you planning to send the patch for review?
Just to mention, in a week or two we plan to do modifications which will enable softfloat on MIPS64.

[wengerxin]

@milanknezevic , Thank you for your answer. Yes, I need CGO to compile docker.
I'm not planning to send the patch for review, I would wait for official merge and then try again.